1. Field of the Invention
The present invention relates to an organic EL display device, which is capable of being effectively subjected to an aging treatment.
2. Description of the Related Art
Organic EL (Electroluminescence) display devices are current drive display devices, which emit light by themselves when a current is supplied to organic EL layers interposed between an anode and a cathode provided so as to be opposite each other. The organic EL display devices are also called organic LEDs because of having characteristics similar to semiconductor light emitting diodes.
The organic EL devices have such a structure that a plurality of anode strips, which is connected to an anode or forms an anode per se, is provided on a glass substrate in parallel with one another, that a plurality of cathode strips, which is connected to a cathode or forms a cathode per se, is provided in parallel with one another and in a direction orthogonal to the anode strips, and that organic EL layers are interposed between both sets of electrode strips. In organic EL devices wherein the anode strips and the cathode strips are provided in a matrix pattern, each of pixels is formed by the intersection of an anode strip and a cathode strip. In other words, the pixels are provided in a matrix pattern. In general, the cathode strips are made of metal, and the anode strips are made of a transparent conductive layer of, e.g., ITO (indium/tin/oxide).
When the organic EL devices wherein the anode strips and the cathode strips are provided in a matrix pattern are driven by passive matrix addressing, the electrodes in one of the set of anode strips or the set of cathodes strips serves as scanning electrodes, and the electrodes in the other set serve as data electrodes. The scanning electrodes are connected to a scanning electrode driving circuit including a constant-voltage circuit in order to drive scanning electrodes at a constant voltage. The data electrodes are connected to a data electrode driving circuit having a constant-current circuit provided at an output stage. In synchronization with scanning, the respective data electrodes are supplied with a current, which is produced in response to a display data for the row corresponding to a selected scanning electrode.
When organic EL display devices having an organic EL device are driven at a constant current, luminance is gradually reducing with the lapse of time. The higher the initial luminance is, the more greatly the degree of luminance decreases. For example, when the initial luminance is twice, the half luminance lifetime is reduced to about half. Additionally, there is caused a phenomenon that luminance varies from pixel to pixel. This is because the longer light emitting time a pixel has, the darker the pixel becomes. This phenomenon is called “image sticking”. When adjacent pixels have a luminance difference of from about 3 to about 5%, the luminance difference can be visually recognized.
When organic EL display devices are energized, the luminance of the organic EL display devices greatly reduces at the initial stage and then is gradually reducing in most cases. In a case wherein the luminance is reducing in that way, when organic EL display devices, which have been driven for some period of time and have had luminance reduced, are newly set at an initial state, the luminance is moderately reducing after the initial state. The treatment wherein organic EL display devices are driven for some period of time to reduce the luminance before the organic EL display devices are practically used (before practical use) is called an aging treatment (hereinbelow, referred to as a lifetime aging treatment).
As the lifetime aging treatment, there is a method wherein the anode strips of an organic EL display device are short-circuited through a lead wire and connected to a voltage applying device, and wherein the cathode strips of the organic EL display device are short-circuited through a lead wire and connected to the voltage applying device (see JP-A-6-20774, paragraphs 0003 and 0006, and FIG. 8). A voltage pulse is applied between the lead wire for connection of the anode strips and the lead wire for connection of the cathode strips from the voltage applying device for a certain period of time.
During fabrication of organic EL display devices, a foreign substance, such as dirt, is mixed into an organic EL layer disposed between an anode strip and a cathode strip, or a projection that is formed on an anode strip intrudes into an organic EL layer in some cases. When electric charges concentrate at such a foreign substance or projection to locally produce heat during practical use of such defective organic EL display devices, decomposition of an organic substance in the organic EL layer progresses. Finally, the organic substance is broken along with a cathode strip, and short-circuit between the cathode strip and an anode strip (interlayer short-circuit) occurs. When the short-circuit occurs, there is caused a phenomenon that a specific pixel fails to emit light during practical use.
In order to avoid the occurrence of such a phenomenon during practical use, an aging treatment is performed wherein a defective portion with a foreign substance mixed therein is preliminarily put in an insulated state as an electrically open state or in a non-conducting state by oxidation as disclosed in, e.g., JP-A-2003-282253, paragraphs 0004 to 0007 (hereinbelow, referred as to a short-circuit aging treatment). The short-circuit aging treatment is performed by applying a d.c. voltage pulse between the anode strips and the cathode strips.